Self-heating of devices in an integrated circuit (IC) is a well-known phenomenon. Devices that dissipate power will heat to a temperature that is determined largely by the thermal resistance of the device. Since many operating characteristics of a device are temperature dependent, self-heating affects device performance.
Two dominant trends in high-speed integrated circuit (IC) design are increasing device speed and decreasing device size. These have been achieved at the expense of higher current density and increased power density. Consequently, devices are operating at elevated temperatures that affect performance. Self-heating is a concern for all circuitry, but it is especially troublesome for precision bias circuitry. Bias circuitry is used to establish steady state or “quiescent” current and voltage levels in other circuitry. For example, in a transconductance cell, the gain is proportional to the bias current through the cell. If the bias circuit used to set the bias current through the gain cell is susceptible to self-heating effects, the performance of the gain cell is adversely impacted.
Bias circuits often include reference cells, which are used to generate reference voltages and currents. A type of reference cell known as a bandgap cell generates reference signals using forward-biased PN junctions, most usually, bipolar transistors having a reliable relationship between collector current (IC) and base-emitter voltage (VBE). For a given value of collector current, VBE is complimentary to absolute temperature (CTAT), i.e., has a negative slope when plotted against absolute temperature. Thus, a single transistor can be used to generate a CTAT reference voltage. However, a reference signal which is either stable with temperature or proportional to absolute temperature (PTAT) is more often needed.
Generating a PTAT signal is commonly accomplished by operating two bipolar transistors at different current densities. It is well known that for two transistors operating at different current densities, the difference in base-emitter voltages is given by:ΔVBE=kT/qln(J1/J2)  Eq. (1)where k is Boltzman's constant, T is absolute temperature, q is the charge of an electron, and J1 and J2 are the current densities of the two transistors. (The expression kT/q is also known as the thermal voltage VT.) Thus, the differential voltage is proportional to absolute temperature (PTAT). The current densities J1 and J2 are typically made unequal by operating the two transistors at the same current and making the emitter areas unequal. Alternatively, the same result could be obtained by setting the emitter areas equal and operating the transistors at unequal currents. Since this cell is based on the difference between the base-emitter voltages of two transistors, it is often referred to as a “ΔVBE” cell.
A stable reference signal can be generated by adding a PTAT signal to a CTAT signal which has a slope of the same magnitude but opposite sign. The classic bandgap circuit for generating a stable reference signal using this technique is shown in FIG. 1. This circuit is known as the Brokaw bandgap cell (named after its inventor, Paul Brokaw, as disclosed in U.S. Pat. No. 3,887,863 and Reissue 30,586).
The bases of transistors Q1 and Q2 are connected together, while the emitters are connected through resistor R2. Transistors Q1 and Q2 are loaded by resistors RC1 and RC2 which are typically selected to be equal. High gain amplifier A drives the bases of Q1 and Q2 so as to equalize the currents IC1 and IC2. The emitter areas A1 and A2 of transistors Q1 and Q2 are unequal, and since IC1=IC2, the transistors operate at different current densities J1 and J2. Thus, according to Eq. (1), VBE for the two transistors are unequal, and the difference voltage ΔVBE appears across resistor R2. The current IP through R2 is therefore given by IP=ΔVBE/R2. However, since the current through both transistors is equal, the current through R1 is twice the current through R2, and the voltage VPTAT across R1 is:VPTAT=2(R1/R2)VTln(A2/A1)  Eq. (2)Thus the voltage across R1 is proportional to absolute temperature since VT is proportional to absolute temperature, i.e., VT=kT/q.
Since VBE for Q1 is CTAT, the output voltage VOUT is the sum of a PTAT voltage across R1 and a CTAT voltage across the base-emitter junction of Q1. By proper selection of component values, the slopes of VPTAT and VCTAT can be made equal in magnitude, and since they are opposite in sign, VOUT will be stable with variations in temperature.
Another circuit used to generate a stable reference signal is shown in FIG. 2. This circuit is similar to that of FIG. 1, but it uses an active load to sense the difference in collector currents more directly. Transistors Q3 and Q4 form a current mirror which tends to force IC1 and IC2 to be equal. Any difference current flows into amplifier A which adjusts the base drive to equalize IC1, and IC2.
The circuits shown in FIGS. 1 and 2 both utilize feedback loops to increase accuracy. A prior art reference cell having multiple loops is shown in FIG. 3. Transistors Q11 and Q12 have emitter areas A1 and A2 respectively. Transistor Q15 supplies equal currents at equilibrium to transistors Q11 and Q12, whose collectors are connected to the emitter of Q15 through resistors R13 and R14 respectively. Current sources CS1 and CS2 set the bias currents through Q13 and Q14. Transistors Q11 and Q12 operate at different current densities J1 and J2, and therefore, different values of VBE. As long as the current densities are maintained at constant values, ΔVBE between Q11 and Q12 will be PTAT and shows up across R11. Thus, the current through R11, designated as IP, is also PTAT.
Resistors R13 and R14 are used to sense the current through Q11 and Q12. Transistors Q13 and Q14 serve two functions. First, they sense the voltage difference at the collectors of Q11 and Q12. Additionally, transistors Q13 and Q14 clamp the voltages at the collectors of Q11 and Q12 respectively at one VBE above the common supply voltage line VGND. This clamping effect reduces the power supply headroom required by transistors Q11 and Q12.
Transistors Q13 and Q15 and resistor R13 form a loop “A” which sets the voltage at the emitter of Q15, thereby maintaining the current through Q11 and Q12. Transistors Q14 and Q16 form a second loop “B” which drives the bases of Q11 and Q12 to balance the currents through the respective transistors. Because Q15 and Q16 are configured as emitter followers, they are both loadable as output nodes.